1. Field of the Invention
The present invention relates to a method of positioning a catheter in a patient's esophagus, a control unit for use with a ventilator, and a computer-readable medium encoded with programming instructions for use in such a control unit.
2. Description of the Prior Art
U.S. Pat. No. 5,671,752 discloses a method and a device for registering the myoelectrical activity of the diaphragm by means of an esophageal catheter having an array of electrodes. Such a signal from an esophageal catheter is prone to disturbances from other myoelectrical signals that are present in the patient's body. For improving the signal-to-noise ratio of such an electromyographic signal cross-correlation of the signals from the different electrode pairs in the catheter is used. Electrodes on opposite sides of the diaphragm but having approximately the same distance to the diaphragm will produce signals that are opposite but substantially equal in magnitude. By subtracting the signals from one electrode from the other the two desired signals will be added, while the noise components of the two signals will substantially cancel each other out.
When a patient is breathing spontaneously but still needs breathing support the myoelectric signal from the diaphragm can be used to control the ventilator. U.S. Pat. Nos. 5,820,560 and 6,588,423 both disclose methods and devices for triggering ventilatory support to a patient using a myoelectrical signal obtained from the diaphragm.
A problem when obtaining a myoelectrical signal from the diaphragm is positioning of the catheter within the patient's esophagus. To obtain a proper signal some of the electrodes should be placed above the diaphragm and some below it. There is a possibility that the catheter will be inserted too far, or not be inserted far enough. In both cases the catheter will detect a weak signal or may not capture any signal at all. The catheter may also capture myoelectrical signals from other muscles instead of, or in addition to, the signal from the diaphragm. Hence, it is difficult to obtain an optimal catheter position and the ventilator may have to work in pneumatic triggering mode if the signal is too weak.
Ensuring the correct positioning of the catheter within the patient is therefore important. Some methods for approximating how far the catheter should be inserted are known. For example the Xiphoid process involves measuring the distance between the bridge of the nose and the earlobe. Based on this distance the distance from the mouth or nose to the esophagogastric junction can be estimated. This is often referred to as NEX, or Nose to ear Xiphoidus measurements. This estimated distance can be used as an initial value when positioning the catheter in the esophagus of a patient. This is, however, only an estimate which may be more or less accurate de-pending on individual variations. This is discussed in the Journal of Advanced Nursing 2007 August; 59(3):274-289. For small children, and especially for premature babies, the distance may be estimated by measuring the circumference of the head. Such a method is disclosed, for example, in WO 2005/115234.
U.S. Pat. No. 6,259,938 discloses method of positioning the catheter by means of depth markings that can be used to gauge the distance the catheter has been inserted into the patient. In addition to the depth markings pressure detecting mechanisms that are spaced apart so that they can detect the esophageal and gastric pressures, to detect when a part of the catheter has reached the stomach.
WO 2006/049787 discloses a system for optically guiding a catheter having a light-emitting means using an external detection device that detects the transdermally projected light emitted by the light-emitting point from within the patient.
These known positioning methods are based on approximations and assumptions, for example, of the actual position of the diaphragm, which will vary from one patient to another. Therefore they cannot assume a precise position of the catheter.